The rationale for the proposed research is the need for strategies to enhance the regeneration of infected bone defects. Infection is one of the most common complications of orthopedic injuries and it has the ability to derail the normal healing process, leading to serious consequences such as amputation. Therefore, the objective of this research is to utilize novel properties of a commonly used antibiotic, clindamycin hydrochloride, to simultaneously treat infection and potentiate bone regeneration in infected bone defects. The fundamental hypothesis for this research is that clindamycin stimulates bone growth through osteogenic differentiation of mesenchymal stem cells (MSCs) and that this effect can reduce or eliminate the dose of bone morphogenetic protein-2 (BMP-2) needed to regenerate large infected bone defects in vivo. The proposed research will be accomplished through two specific aims. In the first specific aim, we will elaborate upon preliminary in vitro and in vivo results tht indicate that clindamycin hydrochloride stimulates the production of bone by 1) evaluating the effects of clindamycin dose and exposure time on MSC differentiation and 2) fabricating clindamycin loaded PLGA microparticles (MPs) and evaluating the effects of release kinetics on MSC differentiation. Activation of signaling proteins will be assessed by Western blot and differentiation will be evaluated by biochemical assay (ALP and calcium) and qRT-PCR (Dlx-5, Runx-2, and ALP). The outcomes from this specific aim will reveal the important proteins and genes involved in clindamycin-stimulated osteogenic differentiation and result in the production of clindamycin-loaded PLGA MPs with release kinetics appropriate for in vivo translation. In the second specific aim, the clindamycin-loaded PLGA MPs developed in Specific Aim 1 will be implanted into infected and non-infected rat critical size rat femoral defects. Dose effects and synergistic potential of clindamycin treatment with and without with BMP-2 will be evaluated. Defects will be inoculated with Staphylococcus aureus, the most common pathogen implicated in long bone osteomyelitis. The femora will be evaluated for volume of regenerated bone, bridging, presence of infection, mechanical properties, and histologic quality of regenerated bone through use of in vivo imaging, microcomputed tomography, mechanical testing, and histology. At the completion of the proposed research, we will have determined how clindamycin affects proteins and genes that can stimulate differentiation of MSCs, developed clindamycin loaded PLGA MPs with appropriate release kinetics to stimulate differentiation, and evaluated the effects of clindamycin delivery with and without BMP-2 in infected critical-size femoral defects. We expect that with the use of clindamycin as both an osteogenic and antimicrobial agent, the amount of BMP-2 needed for union can be reduced or eliminated. Because of the difficulties and expense associated with high doses of BMP-2 for bone regeneration, the proposed research has tremendous potential to reduce risk and cost to patients while improving outcomes.